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........ published in NEWSLETTER # 56
AZOSPIRILLUM AND RELATED MICROORGANISMS - GENETICS, PHYSIOLOGY, ECOLOGY -
by Dr. I. Fendrik, University, Hannover (Germany) Dr. M. Del Gallo, University of Molise, Campobasso (Italy) Dr. J. Vanderleyden, Catholic University, Leuven (Belgium) Dr. M. de Zamaroczy, Pasteur Institute, Paris (France)
Nitrogen fixing rhizosphere bacteria Azospirillum have beneficial effects on plant growth and yield of many crops of agronomic importance. Bacteria of the Genus Azospirillum are from a physiological and molecular point of view, extremely interesting soil microorganisms. Azospirillum has during the past 18 years attracted ever increasing scientific interest for several reasons: it can utilize atmospheric nitrogen and contribute to plant nitrogen nutrition, it can improve the plant nutrient uptake and contribute the balance of the root environment through protection against pathogens, equilibrate nutrient flow and immobilize chemical fertilizers in the soil. They are very versatile also in their carbon and nitrogen metabolism. Azospirillum is also very interesting genetically because it carries in its genomge parts which complement mutations in Rhizobium symbiotic plasmids (p-sym) and Agrobacterium signal transduction genes (hsn.-genes). The ecology of its associations in the rhizosphere and the physiological and morphological effects on inoculated roots constitute a unique model for studying plant bacterial interactions. The subject of the volume will be extended with three newly discovered related microorganisms, particularly endophytes such as Herbaspirillum, Acetobacter and Azoarcus, all highly efficient nitrogen fixing bacteria.
This book (NATO ASI SERIES G37) contains the latest results and exposes some of the new developments. It is of particular interest to note the increasing efforts devoted to the study of bacteria that live inside the plant, the endophytes. From previous research, mainly on Azospirillum-plant interactions, it became evident that plant root surface colonization - as opposed to colonization of the plant root interior - has severe bottlenecks for an efficient exchange of nutrients between bacteria and plants. The question as to how bacteria can infect the plant roots was recognized as a key one. Several research groups have started to investigate the role of mycorrhizal fungi in this infection process. Experimental systems to study this tripartite ecosystem - bacteria, mycorrhizae, plants - are now available, and it is likely that this area will rapidly develop in the near future. Looking at endophytes allows researchers also to draw some parallels with Rhizobium research. As some rhizobia are able to nodulate non- leguminous plants, like Parasponia, an experimental basis to study nodulation of important crop plants is available. Early reports on rice nodulation need further confirmation. Meanwhile, nodulation or Parasponia should be pursued with increasing effort.
The study of Azospirillum-plant interactions will pave the way for the new developments. Using the most advanced tools in molecular genetics, several groups reported on genome analysis, gene identification and gene regulation in Azospirillum. The new findings with respect to nif and gln gene regulation are of particular interest, since Azospirillum is the best model for other plant root associated diazotrophs. The studies on the physical and metabolic interactions between Azospirillum and plant roots reveal some interesting aspects regarding how these partners communicate to each other. Obviously, special interest is given to surface components, such as bacterial exopolysaccharides,
glycoproteins, and plant lectins. Azospirilla, like other plant root associated bacteria, produce phytohormones. This aspect is now extensively studied in Azospirillum brasilense and Enterobacter species. New DNA technologies have also been introduced in the taxonomy and ecological studies of rhizosphere bacteria. These new techniques offer powerful tools for the study of the survival, distribution and the impact of inoculated bacteria. The key paper on Field Applications evaluates the physiological and agronomical aspects of Azospirillum inoculations. Over the last 20 years, in 60 to 70% of experiments worldwide an increase in crop yield could be observed. This increase compensated the inoculation costs and replaced generally about 30% N-fertilizers. A better coordination of field experiments among the researchers and more clarification of the basic mechanisms will be necessary in the future.
Reference books: G37, H17, H45
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